As you can see in the picture above. I can't fully fill the hair with color, why is that?
Close your shapes.
Your example image would seem to reflect what happens when you have a series of disjointed, unconnected paths and then apply a fill to them. If you look, each and every one of your 2 or 3 sided shapes is filled using it's end anchors as the basis for the third side of the fill.
You need to close shapes entirely if you want them to fill as a single object. In reality, open paths with fills can cause all sorts of issues. Not issues directly within Illustrator, but upon output or when files are migrated to another application.
You can simply select all and use the Live Paint Bucket Tool to click and fill areas. The Live Paint Bucket Tool will auto-close shapes when creating fills.